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Currently only the corn kernal is used for making ethanol and the rest of the plant is left in the field, so corn growing returns a lot of carbon to the land. If you read carefully, the above conclusion of carbon deficit only applies if you take the rest of the corn stover and remove it. So current corn growing builds the soil. Anything that uses the plant itself (corn, grass or otherwise) could reduce soil carbon, depending on how much of the plant is used. Normally, when switchgrass yields are talked about, they assume all of it is used, which really takes a lot out of the land. On the otherhand, after centuries of corn growing, farmers know how to sustainably grow corn with crop rotation. They learned this 100 years ago and now are generally are good stewards of the land. If they don't take care of it, they are the ones that suffer.